Background Art of Resin
(Problems Regarding the Conventionally Used Resin)
Plastics such as a polyethylene terephthalate (PET) resin, polyester resin, vinyl chloride resin or polyolefin resin have previously been used for a wide range of uses as molded articles, e.g., containers such as food containers, beverage bottles, cosmetic containers or plant pots.
The majority of these plastics are discarded after use. The plastic wastes have previously been disposed by incineration or landfilling. However, since the wastes generate a great burning energy by incineration, they have problems such as regarding the durability of incinerators caused by a high burning temperature, processing cost by high temperature durable incinerators, and air pollution caused by generation of toxic combustion gas such as carbon monoxide, sulfur compounds, chlorine gas or dioxin. In addition, when the plastic wastes are landfilled, they remain without being decomposed on a semi-permanent basis, and they are accumulated as wastes in a disposal field, thereby causing a social problem that is called a waste problem. Moreover, since the plastic wastes exist as are in the earth, they cause a problem regarding instability of the ground in a landfill site, and there is also a risk that the wastes might affect the natural environment and various types of organisms in the landfill site or the peripheral area.
Thus, to solve these problems, a biodegradable resin has become a focus of attention in these years. The term “biodegradable resin” is used herein to mean a resin, which has physical properties almost equivalent to those of general-purpose plastics during the use as a material, but after the use, is rapidly decomposed by microorganisms in the natural environment such as on the earth, in the earth, in the compost, in the active slurry, or in the water. The resin is decomposed into a fine form, and several types of biodegradable resins are finally converted into carbon dioxide and water.
Other than specific polyester biodegradable resins, blended resin compositions have conventionally been known to satisfy the above described requirements, and examples of such blended resin compositions include a starch-ethylene vinyl alcohol copolymer resin, an ethylene vinyl alcohol copolymer resin-aliphatic polyester resin, and an aliphatic polyester resin-polyolefin resin. These resins or resin compositions are molded into various forms such as a bottle and are in practical use. However, a resin composition, which is excellent in moldability required in its production process, as well as various physical properties required as containers and biodegradability required after being discarded, has not yet been proposed. For example, a resin composition having both biodegradability and heat resistance in a molding process has not yet been accomplished.
(Concerning Polyhydroxyalkanoate (PHA))
By the way, in recent years, as a method for solving the problem regarding environmental contamination caused by wastes such as plastic molded articles, the use of a biodegradable resin synthesized by microorganisms as a molding material has been proposed. Examples of known biodegradable resins derived from microorganisms include polyhydroxyalkanoate (hereinafter referred to as PHA at times) such as a copolymer (hereinafter referred to as PHB/V) of poly-3-hydroxy-n-butyric acid (hereinafter referred to as PHB at times) or 3-hydroxy-n-butyric acid (hereinafter referred to as 3HB at times) and 3-hydroxy-n-valeric acid (hereinafter referred to as 3HV at times), polysaccharide such as bacteria cellulose or Pullulan, and polyamino acid such as poly-γ-glutamic acid or polylysine. In particular, PHA is, as with the conventional plastics, used for various products after undergoing a melt-processing. Further, since PHA is excellent in biodegradability, it is expected that this compound will be applied to soft components for medical use, etc.
It has hitherto been reported that many microorganisms produce poly-3-hydroxybutyric acid (PHB) or other PHAs and accumulate it in the cell. Like conventional plastics, these polymers can be utilized for the production of various products by melt processing or the like. Also, since they are biodegradable, they have an advantage of being completely broken down by microorganisms in the natural world, and by no means remain in natural environment to cause pollution unlike many conventional synthetic polymeric compounds.
It is known that such PHAs produced by microorganisms may have various compositions and structures depending on types of microorganisms used for its production, the composition of culture medium, the conditions for culture and so forth. Researches on how to control such compositions and structures have hitherto chiefly been made from the viewpoint of the improvement in physical properties of PHAs.
(1) Especially, biosynthesis of PHA obtained by polymerization of a monomer units with a relatively simple structure such as 3HB, 3HV, 3-hydroxyhexanoic acid (hereinafter referred to as 3HHx) and 4-hyddroxy-n-butyric acid (hereinafter referred to as 4HB) have been studied, and production using various microorganisms has been reported (Japanese Patent Publications Nos. 6-15604, 7-14352 and 8-19227; Japanese Patent Application Laid-Open Nos. 5-7492, 5-93049, 7-265065 and 9-191893; Japanese Patent No. 2642937 and Appl. Environ. Microbiol., 58(2), 746, 1992).
(2) When, however, broader application of such PHAs produced by microorganisms, e.g., application as functional polymers is taken into account, a PHA in which a substituent other than an alkyl group has been introduced in the side chain, i.e., “unusual PHA” is expected to be very useful. Examples of such a substituent may include those containing aromatic rings (such as a phenyl group and a phenoxy group), and unsaturated hydrocarbons, an ester group, an allyl group, a cyano group, halogenated hydrocarbons and epoxides.
For example, there are reports on production of: PHA containing a phenyl group or its partially substituted group such as PHA containing 3-hydroxy-5-phenylvaleric acid as a unit using 5-phenylvaleric acid as a substrate (Makromol. Chem. Phys., 191, 1957-1965 (1990); Macromolecules, 24, 5256-5260 (1991) and Chirality, 3, 492-494 (1991)), PHA containing 3-hydroxy-5-(4′-tolyl) valeric acid as a unit using 5-(4′-tolyl) valeric acid as a substrate (Macromolecules, 29, 1762-1766 (1996)), and PHA containing 3-hydroxy-5-(2′, 4′-dinitrophenyl) valeric acid and 3-hydroxy-5-(4′-nitrophenyl) valeric acid as a unit using 5-(2′, 4′-dinitrophenyl) valeric acid as a substrate (Macromolecules, 32, 2889-2895 (1999)); PHA containing a phenoxy group or its partially substituted group such as PHA copolymer containing 3-hydroxy-5-phenoxyvaleric acid and 3-hydroxy-9-phenoxynonanoic acid using 11-pheoxyundecanoic acid as a substrate (Macromol. Chem. Phys., 195, 1665-1672 (1994)), PHA containing a 3-hydroxy-4-phenoxybutyric acid unit and a 3-hydroxy-6-phenoxyhexanoic acid unit from 6-phenoxyhexanoic acid, PHA containing a 3-hydroxy-4-phenoxybutyric acid unit, a 3-hydroxy-6-phenoxyhexanoic acid unit, and a 3-hydroxy-8-phenoxyoctanoic acid unit from 8-phenoxyoctanoic acid, and PHA containing a 3-hydroxy-5-phenoxyvaleric acid unit and a 3-hydroxy-7-phenoxyheptanoic acid unit from 11-phenoxyundecanoic acid (Macromolecules, 29, 3432-3435 (1996)). There is also a report (Japanese Patent No. 2989175) on a homopolymer consisting of 3-hydroxy-5-(monofluorophenoxy) pentanoate (3H5(MFP)P) units or 3-hydroxy-5-(difluorophenoxy) pentanoate (3H5(DFP)P) units, and a PHA copolymer containing at least (3H5(MFP)P) units or (3H5(DFP)P) units, of which advantage is to provide stereoregularity and water repellency while maintaining a high melting point and good processability.
Further, studies are conducted on cyano-substituents and nitro-substituents in addition to the fluorine-substituent described above. For example, PHA containing 3-hydroxy-p-cyanophenoxyhexanoic acid or 3-hydroxy-p-nitrophenoxyhexanoic acid as a monomer unit is produced using octanoic acid and p-cyanophenoxyhexanoic acid or p-nitrophenoxyhexanoic acid as substrates (Can. J. Microbiol., 41, 32-43 (1995); and Polymer International, 39, 205-213 (1996)).
These reports are useful in obtaining polymers each having an aromatic ring in the side chain of PHA and having properties derived therefrom unlike general PHA whose side chain contains an alkyl group. Further, as the example of unusual-PHA having a cyclohexyl group, production of PHA from cyclohexylbutyric acid or cyclohexylvaleric acid has been reported (Macromolecules, 30, 1611-1615 (1997)).
(3) Without being confined merely to changes in physical properties, research in a new category is being conducted to produce a PHA having a suitable functional group in the side chain.
For example, a study has been made to produce a PHA having, in a unit thereof, an active group having high reactivity in an addition reaction, such as a bromo group or vinyl group, and to introduce any given functional group into the side chain of the polymer by chemical transformation using the above active group, so as to obtain a multifunctional PHA.
As an example of synthesizing a PHA containing a unit having a thioether (—S—; a sulfanyl linkage), which is expected to provide a high reactivity, Macromolecules, 32, 8315-8318 (1999) reports that Pseudomonas putida strain 27N01 produces a PHA copolymer of 3-hydroxy-5-thiophenoxyvaleric acid (3-hydroxy-5-(phenylsulfanyl)valeric acid) with 3-hydroxy-7-thiophenoxyheptanoic acid (3-hydroxy-7-(phenylsulfanyl)heptanoic acid), using 11-thiophenoxyundecanoic acid (11-(phenylsulfanyl)undecanoic acid) as a substrate.
Macromol. Rapid Commun., 20, 91-94 (1999) has reported that using Pseudomonas oleovorans, a PHA having a bromo group on a side chain thereof is produced, and then the side chain is modified with the thiol product of an acetylated maltose, so as to synthesize PHAs having different solubility and hydrophilicity.
It is reported in Polymer, 41, 1703-1709 (2000) that a change of solubility in solvents has been found such that 3-hydroxyalkanoic acid having diol on the side chain terminal, synthesized by an oxidation reaction using potassium permanganate after producing PHA containing as a monomer unit 3-hydroxyalkenoic acid having an unsaturated bond in the terminal of the side chain terminal using 10-undecenoic acid as a substrate, is rendered soluble in polar solvents such as methanol, acetone-water mixture (80/20, v/v) and dimethylsulfoxide, and insoluble in nonpolar solvents such as chloroform, tetrahydrofuran and acetone.
Likewise, Macromolecules, 31, 1480-1486 (1998) has reported that using Pseudomonas oleovorans, polyester having a vinyl group on a side chain thereof is produced, and then the vinyl group is epoxidized, so as to produce polyester having an epoxy group on a side chain thereof.
Moreover, Polymer, 35, 2090-2097 (1994) has reported that using a vinyl group on the side chain of polyester, a crosslinking reaction is carried out in the polyester molecule, so as to improve the properties of the polyester.
It is reported in Macromolecular chemistry, 4, 289-293 (2001) that an improvement in speed of decomposition has been found for PHA containing 3-hydroxy-10-carboxydecanoic acid as a monomer unit, synthesized by an oxidization cleavage reaction using potassium permanganate after producing PHA containing as a monomer unit 3-hydroxy-10-undecenoic acid using 10-undecenoic acid as a substrate.
At the same time, in order to change the physical properties of a PHA having an active group in its unit and to actually use it as a polymer, the synthesis of a PHA copolymer comprising units other than units having active groups by using microorganisms has been studied. Macromolecules, 25, 1852-1857 (1992) has reported that using Pseudomonas oleovorans, a PHA copolymer comprising a 3-hydroxy-ω-bromoalkanoic acid unit and a straight-chain alkanoic acid unit has been produced in the coexistence of ω-bromoalkanoic acid and n-nonanoic acid, such as 11-bromoundecanoic acid, 8-bromooctanoic acid and 6-bromohexanoic acid.
Thus, into a PHA having, in its units, active groups with high reactivity, such as a bromo or vinyl group, various functional groups can be introduced. Or, chemical transformation can also be performed on such a PHA. Moreover, since a PHA having an active group can be a crosslink point of a polymer, it can be said that such a PHA is extremely effective to achieve multifunctionality of a PHA.